Re: classic problem with emailing groups on MS products
Sorry, but we have decades of evidence that training and 'are you sure' simply don't work. People are too busy thinking about what they are trying to achieve to give any thought to the software that they shouldn't have to know much about.
What might work is for the client side to not accept more than one address in the To or CC fields, only BCC. I suppose there are use cases for allowing people to 'Reply All' but that should be extra work. Not the default. (Hey, that solves another stupid email abuse issue. A Toofer!)
Being a rules engine isn't what people are good at. We have these new-fangled computermabobs for that.